Jetpack Connect: Add REST fallback for connect endpoints

[gravityrail]

If sites have XMLRPC blocked for security reasons, Jetpack will fail to connect.

This PR adds REST versions of the two most critical XMLRPC methods so that we can still connect, even though other things will also fail. This at least allows us to get users started with Jetpack so that we can help them make their site compatible.

To test:

• Block XMLRPC on your server like this (nginx example):

    location = /xmlrpc.php {
        deny all;
        access_log off;
        log_not_found off;
    }

• Attempt to connect
• You should be able to connect! But things like auto-installing VaultPress/Akismet will fail later.

Here's a screenshot of ngrok logged requets during Jetpack Connect.

[roccotripaldi]

This looks good and tests pretty well.

@gravityrail - we may need some small adjustments server-side. But i think this approach makes sense, and I'd be fine merging it into Jetpack.

[beaulebens]

This should be at least 5.3, no? :)